This invention relates to a variable tuning filter in a high frequency circuit.
In a typical high frequency apparatus, operated for instance at 900 MHz, when a desired frequency is selected out of a wide band of several channels, tuning is accomplished by changing the local frequency at the intermediate frequency stage in the signal receiving circuit. However, this method often suffers from the inclusion of interferences such as mutual modulation interference and image interference, because unwanted signals are amplified and mixed in the signal receiving circuit.
In a conventional 1/4.lambda. resonator as shown in FIG. 1, an electrically conductive layer having a pattern as indicated by the oblique lines is formed on a dielectric layer. More specifically, a plurality of strip lines S.sub.1 and S.sub.2 are connected to trimmer capacitors C.sub.T1 and C.sub.T2, and the other ends of the strip lines are connected to a common grounding electrode. In order to obtain the matching of input and output with such a conventional resonator, the input-output parts P.sub.1 and P.sub.2 extended from the strip lines S.sub.1 and S.sub.2 must be positioned carefully.
Therefore, with the conventional resonator the matching of input and output and the improvement of the quality factor in the high frequency band are very difficult to achieve. Because of these drawbacks the employment of such a resonator is not practical.